BRIDGE FATIGUE MYTHS

In this installment of the "Bridge Crossings" series, the author addresses two myths related to bridge fatigue. The first myth applies to categorizing details and the question raised is What if a detail experiences no applied tension? The author responds that some designers prematurely categorize details and that in the 1974 "Interim American Association of State Highway and Transportation Officials (AASHTO) Specifications," details that experience only fluctuating compressive stresses are not considered for fatigue design. The second myth applies to designing for fatigue and the question raised is How many cycles are enough, or is a bridge's fatigue life gone after two million cycles? The author writes that the codewriters who developed the fatigue provisions of the "Load and Resistance Factor Design (LRFD) Specification" wanted the number of cycles for fatigue check to be realistic so bridge evaluators could better comprehend the actual remaining life of a bridge in comparison to the number of cycles used for design.